Metallic elements such as cobalt and nickel are frequently added to gold plating compositions to increase the hardness of the electrodeposited metal; when composition and plating conditions are closely controlled and contamination is minimized, bright deposits are readily obtained over a satisfactory range of current densities. Unfortunately, contamination with various metals frequently occurs and the stability of the bath is affected significantly; moreover, the current efficiency of the bath may be reduced rapidly.
Phosphonic acid chelating agents have long been proposed as components of gold and other metal plating baths to chelate contaminants such as copper and lead. Moreover, it has been recognized that iron contamination can be minimized in baths using a phosphate electrolyte since the phosphate will react with the iron to produce a precipitate.
Exemplary of the baths containing phosphonic acid chelating agents are U.S. Pat. No. 3,770,596 granted Nov. 6, 1973 to Bick et al.; U.S. Pat. No. 3,672,969 to Nobel et al.; U.S. Pat. No. 3,706,634 granted Dec. 19, 1972 to Kowalski; and U.S. Pat. No. 3,904,493 granted Sept. 9, 1975 to Losi et al. Bick et al. U.S. Pat. No. 3,856,638 granted Dec. 24, 1974 is of interest in proposing that the nickel and cobalt be reacted with a phosphonic acid compound and aminoguanidine.
The use of various other organic compounds as chelating agents, brighteners, levelling agents and the like has long been known, and the patent and technical literature contain many compounds and combinations of compounds proposed to offer various advantages. Moreover, buffering agents such as citric acid, boric acid, malic acid and the like have also been proposed to control the pH of the bath.
Despite the various known combinations of compounds that might be added to a plating bath for electrodepositing a hard gold alloy, it has remained a continuing problem to provide a stable bath which would operate at high current efficiency in a wide range of current densities and without requiring a change in bath makeup. Moreover, the industry desires all purpose baths which can be used for rack, barrel and high speed plating applications with reasonable efficiency.
In copending application for U.S. Pat., Ser. No. 027,364, filed Apr. 5, 1979, and entitled "Plating Composition and Method" (now issued as U.S. Pat. No. 4,197,172), applicants have described novel plating baths which produce hard, bright gold deposits at relatively high current efficiencies, which are able to tolerate reasonable amounts of the conventional metal contaminants, and which may be readily prepared and replenished. They also describe a novel plating method, which is simple, relatively trouble-free and is not critical from the standpoint of operating conditions. Notwithstanding the very significant benefits afforded by the foregoing composition and method, yet further improvements, such as may permit broader applicability and extended ranges of operability are, of course, desirable.
Accordingly, it is an object of the present invention to provide a novel gold plating bath which contains a metal constituent, which is stable and efficient over wide ranges of current density, pH values and temperatures, and especially at high current densities, and which may be used for rack, barrel, strip and other high speed applications.
It is also an object to provide such a bath which may be formulated readily and relatively economically and which is highly effective in resisting the effects of copper, lead and iron contamination.
Another object is to provide a novel and highly efficient method for electrodepositing hard, bright gold alloy deposits over wide ranges of current densities, pH values and temperatures, and in various types of plating applications.